Add the function is_trusted(), so developers can check if a variable can be trusted to not contain an Injection Vulnerability.
By trusting integers and literals (strings defined by the programmer), we can provide a lightweight, simple, and very effective way to prevent Injection Vulnerabilities.
It avoids the “false sense of security” that comes with the flawed “Taint Checking” approach, because escaping is very difficult to get right. It's much safer for developers to use parameterised queries, and well-tested libraries.
These libraries require certain sensitive values to only come from the developer; but because it's easy to incorrectly include user values, Injection Vulnerabilities are still introduced by the thousands of developers using these libraries incorrectly. You will notice the linked examples are based on examples found in the Libraries' official documentation, they still “work”, and are typically shorter/easier than doing it correctly (I've found many of them on live websites, and it's why I'm here). A simple Query Builder example being:
$qb->select('u') ->from('User', 'u') ->where('u.id = ' . $_GET['id']); // INSECURE
The “Future Scope” section explains why a dedicated type should come later, and how native functions could use the trusted flag as well.
Injection and Cross-Site Scripting (XSS) vulnerabilities are easy to make, hard to identify, and very common.
We like to think every developer reads the documentation, and would never directly include (inject) user values into their SQL/HTML/CLI - but we all know that's not the case.
It's why these two issues have always been on the OWASP Top 10; a list designed to raise awareness of common issues, ranked on their prevalence, exploitability, detectability, and impact:
| Year | Injection Position | XSS Position |
|---|---|---|
| 2017 - Latest | 1 | 7 |
| 2013 | 1 | 3 |
| 2010 | 1 | 2 |
| 2007 | 2 | 1 |
| 2004 | 6 | 4 |
| 2003 | 6 | 4 |
Google are already using this concept with their Go and Java libraries, and it's been very effective.
Christoph Kern (Information Security Engineer at Google) did a talk in 2016 about Preventing Security Bugs through Software Design (also at USENIX Security 2015), pointing out the need for developers to use libraries (like go-safe-html and go-safesql) to do the encoding, where they only accept strings written by the developer (literals). This ensures the thousands of developers using these libraries cannot introduce Injection Vulnerabilities.
It's been so successful Krzysztof Kotowicz (Information Security Engineer at Google, or “Web security ninja”) is now adding it to JavaScript (details below).
Libraries would be able to use is_trusted() immediately, allowing them to warn developers about Injection Issues as soon as they receive any non-trusted values. Some already plan to implement this, for example:
Propel (Mark Scherer): “given that this would help to more safely work with user input, I think this syntax would really help in Propel.”
RedBean (Gabor de Mooij): “You can list RedBeanPHP as a supporter, we will implement this into the core.”
Psalm (Matthew Brown): “I've just added support for a literal-string type to Psalm: https://psalm.dev/r/9440908f39”
Add the function is_trusted(), where “trusted” is defined as:
No input values are interned; instead an interned string includes:
Any function or instruction that is aware of trusted variables shall produce a trusted string if all input would pass is_trusted(). This includes sprintf(), str_repeat(), str_pad(), implode(), join(), array_pad(), and array_fill().
is_trusted('Example'); // true $a = 'Hello'; $b = 'World'; is_trusted($a); // true is_trusted($a . $b); // true is_trusted("Hi $b"); // true is_trusted($_GET['id']); // false is_trusted(sprintf('Hi %s', $_GET['name'])); // false is_trusted('/bin/rm -rf ' . $_GET['path']); // false is_trusted('<img src=' . htmlentities($_GET['src']) . ' />'); // false is_trusted('WHERE id = ' . $db->real_escape_string($_GET['id'])); // false function example($input) { if (!is_trusted($input)) { throw new Exception('Non-trusted value detected!'); } return $input; } example($a); // OK example(example($a)); // OK, still the same trusted value. example(strtoupper($a)); // Exception thrown.
Have a play with it on 3v4l.org
How it can be used by libraries - Notice how this example library just raises a warning, to simply let the developer know about the issue, without breaking anything. And it provides an “unsafe_value” value-object to bypass the is_trusted() check, but none of the examples need to use it (can be useful as a temporary thing, but there are much safer/better solutions, which developers are/should already be using).
Taint checking is flawed, isn't this the same?
It is not the same. Taint Checking incorrectly assumes the output of an escaping function is “safe” for a particular context. While it sounds reasonable in theory, the operation of escaping functions, and the context for which their output is safe, is very hard to define and led to a feature that is both complex and unreliable.
$sql = 'SELECT * FROM users WHERE id = ' . $db->real_escape_string($id); // INSECURE $html = "<img src=" . htmlentities($url) . " alt='' />"; // INSECURE $html = "<a href='" . htmlentities($url) . "'>..."; // INSECURE
All three examples would be incorrectly considered “safe” (untainted). The first two need the values to be quoted. The third example, htmlentities() does not escape single quotes by default before PHP 8.1 (fixed), and it does not consider the issue of 'javascript:' URLs.
In comparison, is_trusted() doesn't have an equivalent of untaint(), or support escaping. Instead PHP will set the trusted flag, and as soon as the value has been manipulated or includes anything that is not trusted (e.g. user data), the trusted flag is lost.
This allows libraries to use is_trusted() to check the sensitive values they receive from the developer. Then it's up to the library to handle the escaping (if it's even needed). The “Future Scope” section notes how native functions will be able to use the trusted flag as well.
Why not educate everyone?
You can't - developer training simply does not scale, and mistakes still happen.
We cannot expect everyone to have formal training, know everything from day 1, and consider programming a full time job. We want new programmers, with a variety of experiences, ages, and backgrounds. Everyone should be guided to do the right thing, and notified as soon as they make a mistake (we all make mistakes). We also need to acknowledge that many programmers are busy, do copy/paste code, don't necessarily understand what it does, edit it for their needs, then simply move on to their next task.
Why not use static analysis?
It will never be used by most developers.
I still agree with Tyson Andre, you should use Static Analysis, but it's an extra step that most programmers cannot be bothered to do, especially those who are new to programming (its usage tends to be higher among those writing well-tested libraries).
Also, these tools currently focus on other issues (type checking, basic logic flaws, code formatting, etc), rarely attempting to address Injection Vulnerabilities. Those that do are often incomplete, need sinks specified on all library methods (unlikely to happen), and are not enabled by default. For example, Psalm, even in its strictest errorLevel (1), and running --taint-analysis (rarely used), will not notice the missing quote marks in this SQL, and incorrectly assume it's safe:
$db = new mysqli('...'); $id = (string) ($_GET['id'] ?? 'id'); // Keep the type checker happy. $db->prepare('SELECT * FROM users WHERE id = ' . $db->real_escape_string($id)); // INSECURE
What about the performance impact?
These stats from an early version of the implementation (new tests will be completed soon).
Máté Kocsis has created a php benchmark to replicate the old Intel Tests, and the preliminary testing on this implementation has found a 0.124% performance hit for the Laravel Demo app, and 0.161% for Symfony (rounds 4-6, which involved 5000 requests). These tests do not connect to a database, as the variability introduced makes it impossible to measure that low of a difference.
The full stress-test is 3.719% when running this concat test, but this is not representative of a typical PHP script (it's not normal to concatenate 4 strings, 5 million times, with no other actions).
Joe Watkins has also noted that further optimisations are possible (the implementation has focused on making it work).
Is string concatenation supported?
Yes. The trusted flag is preserved when two trusted values are concatenated; this makes it easier to use is_trusted(), especially by developers that use concatenation for their SQL/HTML/CLI/etc.
Previously we tried a version that only supported concatenation at compile-time (not run-time), to see if it would reduce the performance impact even further. The idea was to require everyone to use special trusted_concat() and trusted_implode() functions, which would raise exceptions to highlight where mistakes were made. These two functions can still be implemented by developers themselves (see Support Functions below), as they can be useful; but requiring everyone to use them would have required big changes to existing projects, and exceptions are not a graceful way of handling mistakes.
Performance wise, my simplistic testing found there was still a small impact without run-time concat:
Laravel Demo App: +0.30% with, vs +0.18% without. Symfony Demo App: +0.06% with, vs +0.06% without. My Concat Test: +4.36% with, vs +2.23% without. - Website with 22 SQL queries: Inconclusive, too variable.
(This is because concat_function() in “zend_operators.c” uses zend_string_extend() which needs to remove the trusted flag. Also “zend_vm_def.h” does the same; and supports a quick concat with an empty string (x2), which would need its flag removed as well).
And by supporting both forms of concatenation, it makes it easier for developers to understand (many are not aware of the difference).
Why don't you support string splitting?
In short, we can't find any real use cases (security features should try to keep the implementation as simple as possible).
Also, the security considerations are different. Concatenation joins known/fixed units together, whereas if you're starting with a trusted string, and the program allows the Evil-User to split the string (e.g. setting the length in substr), then they get considerable control over the result (it creates an untrusted modification).
These are unlikely to be written by a programmer, but consider these:
$length = ($_GET['length'] ?? -5); $url = substr('https://example.com/js/a.js?v=55', 0, $length); $html = substr('<a href="#">#</a>', 0, $length);
If that URL was used in a Content-Security-Policy, then it's necessary to remove the query string, but as more of the string is removed, the more resources can be included (“https:” basically allows resources from anywhere). With the HTML example, moving from the tag content to the attribute can be a problem (technically the HTML Templating Engine should be fine, but unfortunately libraries like Twig are not currently context aware, so you need to change from the default 'html' encoding to explicitly using 'html_attr' encoding).
Or in other words; trying to determine if the trusted flag should be passed through functions like substr() is difficult. Having a security feature be difficult to reason about, gives a much higher chance of mistakes.
Krzysztof Kotowicz has confirmed that, at Google, with “go-safe-html”, splitting is explicitly not supported because it “can cause issues”; for example, “arbitrary split position of a HTML string can change the context”.
What about an undefined number of parameters, e.g. WHERE id IN (?, ?, ?)?
If the values are explicitly cast to integers, there is no need to make a change.
That said, ideally you would still follow the advice from Levi Morrison, PDO Execute, and Drupal Multiple Arguments, and implement as such:
$sql = 'WHERE id IN (' . join(',', array_fill(0, count($ids), '?')) . ')';
Or, if you prefer to use concatenation:
$sql = '?'; for ($k = 1; $k < $count; $k++) { $sql .= ',?'; }
How can this work with Table and Field names in SQL, which cannot use parameters?
They are often in variables written as literal strings anyway (so no changes needed); and if they are dependent on user input, in most cases you can (and should) use literals:
$order_fields = [ 'name', 'created', 'admin', ]; $order_id = array_search(($_GET['sort'] ?? NULL), $order_fields); $sql .= ' ORDER BY ' . $order_fields[$order_id];
By using an allow-list, we ensure the user (attacker) cannot use anything unexpected.
How does this work in cases where you can't use trusted values?
For example Dennis Birkholz noted that some Systems/Frameworks currently define some variables (e.g. table name prefixes) without the use of a literal (e.g. ini/json/yaml). And Larry Garfield noted that in Drupal's ORM “the table name itself is user-defined” (not in the PHP script).
While most systems can use trusted values entirely, these special non-trusted values should still be handled separately (and carefully). This approach allows the library to ensure the majority of the input (SQL) is trusted, and then it can consistently check/escape those special values (e.g. does it match a valid table/field name, which can be included safely).
How this can be done with aliases, or the example Query Builder.
Could libraries use is_trusted() internally?
Yes, they could.
It would be fantastic if they did use additional is_trusted() checks after receiving the values from developers (it ensures the library hasn't introduced a vulnerability either); but this isn't a priority, simply because libraries are rarely the source of Injection Vulnerabilities.
That said, consider the Drupalgeddon vulnerability; where $db->expandArguments() allowed unsafe/non-trusted values to be used as placeholders with IN (:arg_0, :arg_1). By using something like the example Query Builder, is_trusted() could have been used to check the raw SQL, and then the non-trusted field/parameter names would be checked and applied separately.
Zend also had a couple of issues with ORDER BY, where it didn't check the inputs either (1/2).
Can you support Integer values?
Yes, support for integers is now included.
It was noted by Matthew Brown (and others) that a lot of existing code and tutorials uses integers directly, and they do not cause a security issue.
We tried to flag integers defined in the source code, in the same way we are doing with strings. Unfortunately it would require a big change to add a trusted flag on integers. Changing how integers work internally would have made a big performance impact, and potentially affected every part of PHP (including extensions).
That said, while it's not as philosophically pure, we can still trust all integers in regards to Injection Vulnerabilities, no matter where they came from.
We have not found any issues with allowing integers in SQL, HTML, CLI; and other contexts as well (e.g. preg, mail additional_params, XPath query, and even eval).
We could not find any character encoding issues (The closest we could find was EBCDIC, an old IBM character encoding, which encodes the 0-9 characters differently; which anyone using it would need to re-encode either way, and EBCDIC is not supported by PHP).
We also checked what would happen if a 64bit PHP server sent a large number to a 32bit database, but that's not an issue either, because the number is being encoded as characters in a string, so that's also fine.
Why don't you support Boolean/Float values?
It's a very low value feature, and we cannot be sure of the security implications.
For example, the value you put in often is not always the same as what you get out:
var_dump((string) true); // "1" var_dump((string) false); // "" var_dump(2.3 * 100); // 229.99999999999997 setlocale(LC_ALL, 'de_DE.UTF-8'); var_dump(sprintf('%.3f', 1.23)); // "1,230" // Note the comma, which can be bad for SQL. // Pre 8.0 this also happened with string casting.
Why is it called is_trusted()?
First, there is no perfect name.
We did start with is_literal() as a placeholder name (at a time we only trusted literals). This name wasn't perfect, but it would have allowed developers to search and get an idea of what a literal was. When integer values were deemed necessary to help adoption, the name became more of a problem. We also need to keep to a single word name (to support a dedicated type in the future). This is where is_trusted() and is_known() was proposed. We had a vote on the name, which gave us a 18 to 3 result in favour of is_trusted().
What about other support functions?
We did consider trusted_concat() and trusted_implode() functions (see String Concatenation above), but these can be userland functions:
function trusted_implode($separator, $array) { $return = implode($separator, $array); if (!is_trusted($return)) { // You will probably only want to raise // an exception on your development server. throw new Exception('Non-trusted value detected!'); } return $return; } function trusted_concat(...$a) { return trusted_implode('', $a); }
Developers can use these to help identify exactly where they made a mistake, for example:
$sortOrder = 'ASC'; // 300 lines of code, or multiple function calls $sql .= ' ORDER BY name ' . $sortOrder; // 300 lines of code, or multiple function calls $db->query($sql);
If a developer changed the literal 'ASC' to $_GET['order'], the error would be noticed by $db->query(), but it's not clear where the non-trusted value was introduced. Whereas, if they used trusted_concat(), that would raise an exception much earlier, and highlight exactly where the mistake happened:
$sql = trusted_concat($sql, ' ORDER BY name ', $sortOrder);
Why not support other string functions?
Like String Splitting, we can't find any real use cases, and don't want to make this complicated. For example strtoupper() might be reasonable, but we will need to consider how it would be used (good and bad), and check for any oddities (e.g. output varying based on the current locale). Also, functions like str_shuffle() create unpredictable results.
Does this mean the value is completely safe?
While these values can be trusted to not contain an Injection Vulnerability, they cannot be completely safe from every kind of issue, For example:
$cli = 'rm -rf ?'; // RISKY $sql = 'DELETE FROM my_table WHERE my_date >= ?'; // RISKY
The parameters could be set to “/” or “0000-00-00”, which can result in deleting a lot more data than expected.
There's no single RFC that can completely solve all developer errors, but this takes one of the biggest ones off the table.
What if I really want to mark a value as trusted?
This implementation does not provide a way for a developer to mark anything they want as trusted. This is on purpose. We do not want to recreate the biggest flaw of Taint Checking. It would be very easy for a naive developer to mark all escaped values as trusted (wrong).
That said, we do not pretend there aren't ways around this (e.g. using var_export), but doing so is clearly the developer doing something wrong. We want to provide safety rails, but there is nothing stopping the developer from jumping over them if that's their choice.
Extensions create and manipulate strings, won't this break the flag on strings?
Strings have multiple flags already that are off by default - this is the correct behaviour when extensions create their own strings (should not be flagged as trusted). If an extension is found to be already using the flag we're using for is_trusted (unlikely), that's the same as any new flag being introduced into PHP, and will need to be updated in the same way.
Why don't you use the Reflection API?
This allows you to “introspect classes, interfaces, functions, methods and extensions”; it's not currently set up for object methods to inspect the code calling it. Even if that was to be added (unlikely), it could only check if the trusted value was defined there, it couldn't handle variables (tracking back to their source), nor could it provide any future scope for a dedicated type, nor could native functions work with this (see “Future Scope”).
Why does the output from chr() appear as trusted?
This was noticed by Claude Pache, and on a technical level is due to the use of Interned Strings, an optimisation used by RETURN_CHAR that re-uses single character values. It's effectively the same as calling sprintf('%c', $i), which is also not an issue, as the developer is choosing to do this.
Go programs can use “ScriptFromConstant” to express the concept of a “compile time constant” (more details).
Java can use Error Prone with @CompileTimeConstant to ensure method parameters can only use “compile-time constant expressions”.
JavaScript is getting isTemplateObject, for “Distinguishing strings from a trusted developer from strings that may be attacker controlled” (intended to be used with Trusted Types).
Perl has a Taint Mode, via the -T flag, where all input is marked as “tainted”, and cannot be used by some methods (like commands that modify files), unless you use a regular expression to match and return known-good values (where regular expressions are easy to get wrong).
There is a Taint extension for PHP by Xinchen Hui, and a previous RFC proposing it be added to the language by Wietse Venema.
And there is the Automatic SQL Injection Protection RFC by Matt Tait (this RFC uses a similar concept of the SafeConst). When Matt's RFC was being discussed, it was noted:
All of these concerns have been addressed by is_trusted().
I also agree with Scott Arciszewski, “SQL injection is almost a solved problem [by using] prepared statements”, where is_trusted() is essential for identifying the mistakes developers are still making.
No known BC breaks, except for code-bases that already contain the userland function is_trusted() which is unlikely.
PHP 8.1
None known
None known
None known
None
1) As noted by someniatko and Matthew Brown, having a dedicated type would be useful in the future, as “it would serve clearer intent”, which can be used by IDEs, Static Analysis, etc. It was agreed we would add this type later, via a separate RFC, so this RFC can focus on the trusted flag, and provide libraries a simple backwards-compatible function, where they can decide how to handle non-trusted values.
2) As noted by MarkR, the biggest benefit will come when this flag can be used by PDO and similar functions (mysqli_query, preg_match, exec, etc).
However, first we need libraries to start using is_trusted() to check their inputs. The library can then do their thing, and apply the appropriate escaping, which can result in a value that no longer has the trusted flag set, but should still be trusted.
With a future RFC, we could potentially introduce checks for the native functions. For example, if we use the Trusted Types concept from JavaScript (which protects 60+ Injection Sinks, like innerHTML), the libraries create a stringable object as their output. These objects can be added to a list of trusted objects for the relevant native functions. The native functions could then warn developers when they do not receive a value with the trusted flag, or one of the trusted objects. These warnings would not break anything, they just make developers aware of the mistakes they have made, and we will always need a way of switching them off entirely (e.g. phpMyAdmin).
Accept the RFC. Yes/No
N/A
N/A
